Support pruning properties by build release

Adds a general mechanism for pruning selected sdk member properties
(i.e. setting their fields to their zero value) and uses that to prune
any properties that do not support a specified target build release.

Follow up changes will use that to allow building an sdk snapshot that
is compatible with previous release S.

Bug: 197842263
Test: m nothing
Change-Id: Ib949a9cfe85fff30f86228eeb15d3a45c073b037

sdk/build_release.go

@@ -16,6 +16,7 @@ package sdk
 
 import (
 	"fmt"
+	"reflect"
 	"strings"
 )
 
@@ -158,3 +159,166 @@ func parseBuildReleaseSet(specification string) (*buildReleaseSet, error) {
 
 	return set, nil
 }
+
+// Given a set of properties (struct value), set the value of a field within that struct (or one of
+// its embedded structs) to its zero value.
+type fieldPrunerFunc func(structValue reflect.Value)
+
+// A property that can be cleared by a propertyPruner.
+type prunerProperty struct {
+	// The name of the field for this property. It is a "."-separated path for fields in non-anonymous
+	// sub-structs.
+	name string
+
+	// Sets the associated field to its zero value.
+	prunerFunc fieldPrunerFunc
+}
+
+// propertyPruner provides support for pruning (i.e. setting to their zero value) properties from
+// a properties structure.
+type propertyPruner struct {
+	// The properties that the pruner will clear.
+	properties []prunerProperty
+}
+
+// gatherFields recursively processes the supplied structure and a nested structures, selecting the
+// fields that require pruning and populates the propertyPruner.properties with the information
+// needed to prune those fields.
+//
+// containingStructAccessor is a func that if given an object will return a field whose value is
+// of the supplied structType. It is nil on initial entry to this method but when this method is
+// called recursively on a field that is a nested structure containingStructAccessor is set to a
+// func that provides access to the field's value.
+//
+// namePrefix is the prefix to the fields that are being visited. It is "" on initial entry to this
+// method but when this method is called recursively on a field that is a nested structure
+// namePrefix is the result of appending the field name (plus a ".") to the previous name prefix.
+// Unless the field is anonymous in which case it is passed through unchanged.
+//
+// selector is a func that will select whether the supplied field requires pruning or not. If it
+// returns true then the field will be added to those to be pruned, otherwise it will not.
+func (p *propertyPruner) gatherFields(structType reflect.Type, containingStructAccessor fieldAccessorFunc, namePrefix string, selector fieldSelectorFunc) {
+	for f := 0; f < structType.NumField(); f++ {
+		field := structType.Field(f)
+		if field.PkgPath != "" {
+			// Ignore unexported fields.
+			continue
+		}
+
+		// Save a copy of the field index for use in the function.
+		fieldIndex := f
+
+		name := namePrefix + field.Name
+
+		fieldGetter := func(container reflect.Value) reflect.Value {
+			if containingStructAccessor != nil {
+				// This is an embedded structure so first access the field for the embedded
+				// structure.
+				container = containingStructAccessor(container)
+			}
+
+			// Skip through interface and pointer values to find the structure.
+			container = getStructValue(container)
+
+			defer func() {
+				if r := recover(); r != nil {
+					panic(fmt.Errorf("%s for fieldIndex %d of field %s of container %#v", r, fieldIndex, name, container.Interface()))
+				}
+			}()
+
+			// Return the field.
+			return container.Field(fieldIndex)
+		}
+
+		zeroValue := reflect.Zero(field.Type)
+		fieldPruner := func(container reflect.Value) {
+			if containingStructAccessor != nil {
+				// This is an embedded structure so first access the field for the embedded
+				// structure.
+				container = containingStructAccessor(container)
+			}
+
+			// Skip through interface and pointer values to find the structure.
+			container = getStructValue(container)
+
+			defer func() {
+				if r := recover(); r != nil {
+					panic(fmt.Errorf("%s for fieldIndex %d of field %s of container %#v", r, fieldIndex, name, container.Interface()))
+				}
+			}()
+
+			// Set the field.
+			container.Field(fieldIndex).Set(zeroValue)
+		}
+
+		if selector(name, field) {
+			property := prunerProperty{
+				name,
+				fieldPruner,
+			}
+			p.properties = append(p.properties, property)
+		} else if field.Type.Kind() == reflect.Struct {
+			// Gather fields from the nested or embedded structure.
+			var subNamePrefix string
+			if field.Anonymous {
+				subNamePrefix = namePrefix
+			} else {
+				subNamePrefix = name + "."
+			}
+			p.gatherFields(field.Type, fieldGetter, subNamePrefix, selector)
+		}
+	}
+}
+
+// pruneProperties will prune (set to zero value) any properties in the supplied struct.
+//
+// The struct must be of the same type as was originally passed to newPropertyPruner to create this
+// propertyPruner.
+func (p *propertyPruner) pruneProperties(propertiesStruct interface{}) {
+	structValue := reflect.ValueOf(propertiesStruct)
+	for _, property := range p.properties {
+		property.prunerFunc(structValue)
+	}
+}
+
+// fieldSelectorFunc is called to select whether a specific field should be pruned or not.
+// name is the name of the field, including any prefixes from containing str
+type fieldSelectorFunc func(name string, field reflect.StructField) bool
+
+// newPropertyPruner creates a new property pruner for the structure type for the supplied
+// properties struct.
+//
+// The returned pruner can be used on any properties structure of the same type as the supplied set
+// of properties.
+func newPropertyPruner(propertiesStruct interface{}, selector fieldSelectorFunc) *propertyPruner {
+	structType := getStructValue(reflect.ValueOf(propertiesStruct)).Type()
+	pruner := &propertyPruner{}
+	pruner.gatherFields(structType, nil, "", selector)
+	return pruner
+}
+
+// newPropertyPrunerByBuildRelease creates a property pruner that will clear any properties in the
+// structure which are not supported by the specified target build release.
+//
+// A property is pruned if its field has a tag of the form:
+//     `supported_build_releases:"<build-release-set>"`
+// and the resulting build release set does not contain the target build release. Properties that
+// have no such tag are assumed to be supported by all releases.
+func newPropertyPrunerByBuildRelease(propertiesStruct interface{}, targetBuildRelease *buildRelease) *propertyPruner {
+	return newPropertyPruner(propertiesStruct, func(name string, field reflect.StructField) bool {
+		if supportedBuildReleases, ok := field.Tag.Lookup("supported_build_releases"); ok {
+			set, err := parseBuildReleaseSet(supportedBuildReleases)
+			if err != nil {
+				panic(fmt.Errorf("invalid `supported_build_releases` tag on %s of %T: %s", name, propertiesStruct, err))
+			}
+
+			// If the field does not support tha target release then prune it.
+			return !set.contains(targetBuildRelease)
+
+		} else {
+			// Any untagged fields are assumed to be supported by all build releases so should never be
+			// pruned.
+			return false
+		}
+	})
+}

sdk/build_release_test.go

@@ -98,3 +98,88 @@ func TestBuildReleaseSetContains(t *testing.T) {
 		android.AssertBoolEquals(t, "set does not contain T", false, set.contains(buildReleaseT))
 	})
 }
+
+func TestPropertyPrunerInvalidTag(t *testing.T) {
+	type brokenStruct struct {
+		Broken string `supported_build_releases:"A"`
+	}
+	type containingStruct struct {
+		Nested brokenStruct
+	}
+
+	t.Run("broken struct", func(t *testing.T) {
+		android.AssertPanicMessageContains(t, "error", "invalid `supported_build_releases` tag on Broken of *sdk.brokenStruct: unknown release \"A\"", func() {
+			newPropertyPrunerByBuildRelease(&brokenStruct{}, buildReleaseS)
+		})
+	})
+
+	t.Run("nested broken struct", func(t *testing.T) {
+		android.AssertPanicMessageContains(t, "error", "invalid `supported_build_releases` tag on Nested.Broken of *sdk.containingStruct: unknown release \"A\"", func() {
+			newPropertyPrunerByBuildRelease(&containingStruct{}, buildReleaseS)
+		})
+	})
+}
+
+func TestPropertyPrunerByBuildRelease(t *testing.T) {
+	type nested struct {
+		F1_only string `supported_build_releases:"F1"`
+	}
+
+	type testBuildReleasePruner struct {
+		Default      string
+		S_and_T_only string `supported_build_releases:"S-T"`
+		T_later      string `supported_build_releases:"T+"`
+		Nested       nested
+	}
+
+	input := testBuildReleasePruner{
+		Default:      "Default",
+		S_and_T_only: "S_and_T_only",
+		T_later:      "T_later",
+		Nested: nested{
+			F1_only: "F1_only",
+		},
+	}
+
+	t.Run("target S", func(t *testing.T) {
+		testStruct := input
+		pruner := newPropertyPrunerByBuildRelease(&testStruct, buildReleaseS)
+		pruner.pruneProperties(&testStruct)
+
+		expected := input
+		expected.T_later = ""
+		expected.Nested.F1_only = ""
+		android.AssertDeepEquals(t, "test struct", expected, testStruct)
+	})
+
+	t.Run("target T", func(t *testing.T) {
+		testStruct := input
+		pruner := newPropertyPrunerByBuildRelease(&testStruct, buildReleaseT)
+		pruner.pruneProperties(&testStruct)
+
+		expected := input
+		expected.Nested.F1_only = ""
+		android.AssertDeepEquals(t, "test struct", expected, testStruct)
+	})
+
+	t.Run("target F1", func(t *testing.T) {
+		testStruct := input
+		pruner := newPropertyPrunerByBuildRelease(&testStruct, buildReleaseFuture1)
+		pruner.pruneProperties(&testStruct)
+
+		expected := input
+		expected.S_and_T_only = ""
+		android.AssertDeepEquals(t, "test struct", expected, testStruct)
+	})
+
+	t.Run("target F2", func(t *testing.T) {
+		testStruct := input
+		pruner := newPropertyPrunerByBuildRelease(&testStruct, buildReleaseFuture2)
+		pruner.pruneProperties(&testStruct)
+
+		expected := input
+		expected.S_and_T_only = ""
+		expected.Nested.F1_only = ""
+		android.AssertDeepEquals(t, "test struct", expected, testStruct)
+	})
+}